Traffic Advisory Leaflet 9/99 and DETR Circular 05/00 provide local traffic authorities with guidance on when and where to introduce 20mph zones. Both documents recognise the key to an effective 20mph zone is that it is self-enforcing through regular traffic calming measures which reduce speeds to 20mph or less without the need for police enforcement.
This message was reinforced in the Department's new guidance on setting local speed limits, published in August this year, which further encourages implementation of 20mph zones.